Heavy-metal Pollution Affects Daphnia Fitness by Altering Diversity of the Gut Microbiota

The role of gut microbiota in shaping host fitness is already well established. However, it remains unclear to what extent the gut microbiota influences host fitness in the presence of environmental stressors. Here, we tested the hypothesis that responses of water flea Daphnia to the heavy metal nickel are mediated by gut microbiota. Germ-free D. magna exhibited somewhat lower fitness than did those with gut microbiota transplant. Among germ-free Daphnia, those that were exposed to heavy metals did not differ in fitness from unexposed Daphnia. In contrast, when incubated with their donors' gut microbiota, initially germ-free D. magna continuously exposed to nickel for 21 days showed a significantly lower survival rate than those not exposed to nickel. We detected a reduced set of microbes in the formerly germ-free Daphnia in the presence of nickel. Transcriptomic analysis of Daphnia showed that expression/regulation of genes related to oxygen transport, chitin metabolism, and detoxification changed in response to the reduced gut microbiomes acquired in the presence of nickel. Our findings show that the toxic effects of heavy metal led to a reduced diversity of gut microbiota in Daphnia and can thus affect host fitness.